In a conventional single shaft gas turbine engine, a compressor introduces air into a combustion chamber in which the air is mixed with the burning fuel to produce gases that drive a turbine. The turbine drives a load consisting of the compressor and an external load. In a dual shaft gas turbine, the compressor is driven by a turbine that is separate from the load turbine. The two turbines are not mechanically connected. They are only gas dynamically connected. The gases from the first turbine pass through the second turbine after leaving the first turbine. The compressor is usually driven by the high pressure turbine with the combination of the compressor and turbine being referred to as the gas generator. However, schemes in which the compressor is driven by the low pressure turbine are also known. To simplify the following discussion, a single shaft turbine will be used; however, it will be apparent to those skilled in the art that the teachings of the present invention may be equally applied to a dual turbine configuration. The efficiency of such a turbine design improves with increasing operating temperatures; however, there is a limit to the operating temperature dictated by the temperature at which the turbine blades and related systems fail.
To further increase the efficiency of the engine, the energy that is discarded in the exhaust gases from the turbine must be reclaimed. Schemes in which the exhaust gases are used to heat water in a boiler to generate steam for a steam turbine are known to the art. The efficiency of the steam turbine is determined by the temperature of the steam which, in turn, is determined by the temperature of the exhaust gases leaving the gas turbine. Since the exhaust gases are typically at a temperature of 1000.degree. F., prior art systems utilize steam turbines that operate at temperatures of 1000.degree. F. or less. Since the efficiency of the steam turbine cycle is determined by the temperature of the input steam, any increase in the steam inlet temperature without changing the exhaust temperature will further improve the efficiency of the combined engine.
As noted above, to maintain the temperature below this maximum temperature of the turbine blades, the fuel to air ratio in the combustion chamber is maintained below the point at which stoichiometric combustion of the fuel is achieved. The additional air maintains the gases below the maximum operating temperature. Unfortunately, the energy needed to compress this additional air reduces the overall efficiency of the engine.
These observations have led to gas turbine designs in which steam and/or water is injected into the combustion system. For example, Dah Yu Cheng (U.S. Pat. Nos. 3,978,661, 4,128,994 and 4,297,841) recognized that steam addition to the Brayton cycle can significantly increase the power and efficiency of the engine provided heat is recovered from the exhaust gases. The power generated by the drive turbine at any given temperature is determined by the specific heat of the gases expanding through the turbine. Since steam has about twice the specific heat of air, the use of steam as the coolant significantly improves the power that can be generated by the turbine.
Unfortunately, the amount of heat that leaves the system in the exhaust gases also increases when steam is used. The exhaust gases generated in a steam injected engine leave at a higher temperature and have a higher specific heat. Hence, in the absence of some form of heat recovery system, the overall efficiency of the engine decreases. Cheng used a heat recovery boiler to recover the heat from the exhaust gases of the turbine to produce steam. Because of the pinch point limitation on the operating pressure of the heat recovery boiler, and hence the operating pressure ratio of the turbine, the maximum achievable efficiency was limited in this system. Patton and Shouman (U.S. Pat. No. 4,841,721) solved the pinch point problem by operating the combustor of the gas turbine at a pressure above the supercritical pressure of water. They replaced the boiler by a series of regenerative feed water heaters. Shouman (U.S. Pat. No. 5,491,968) describes a combustion system composed of a wet oxidation reactor to which is added, in series, a second stage combustor to produce the desired turbine inlet temperature. This combustion system replaces the conventional gas turbine combustor when a wet oxidation reactor is used.
Broadly, it is the object of the present invention to provide an improved heat recovery system for use in a gas turbine engine system.
It is a further object of the present invention to provide a heat recovery system that improves the efficiency of water/steam injected gas turbine systems.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.